Tsunami is a series of ocean waves generated by any rapid large-scale disturbance of the sea water. Most tsunamis are generated by earthquakes, but they may also be caused by volcanic eruptions, landslides, undersea slumps or meteor impacts.

The waves radiate outward in all directions from the disturbance and can propagate across entire ocean basins.  Tsunami waves are distinguished from ordinary ocean waves by their great length between peaks. The speed at which tsunamis travel depends on the ocean depth. A tsunami can exceed 500 mph in the deep ocean but slows to 20 or 30 mph in the shallow water near land.

In the deep ocean, a tsunami is barely noticeable and will only cause a small and slow rising and falling of the sea surface as it passes. Only as it approaches land does a tsunami become a hazard. As the tsunami approaches land and shallow water, the waves slow down and become compressed, causing them to grow in height. In the best of cases, the tsunami comes onshore like a quickly rising tide and causes a gentle flooding of low-lying coastal areas.

(Click here to view a high resolution version of NOAA image of a Deep-ocean Assessment and Reporting of Tsunamis (DART) buoy being deployed in the Pacific Ocean from the NOAA ship Ronald H. Brown in October 1999.  

The Pacific Basin—the area served by the existing tsunami warning system established by the Pacific rim countries and operated by NOAA in Hawaii.

The Pacific Basin tsunami warning system did not detect a tsunami in the Indian Ocean since there are no buoys in place here. Even without a way to detect whether a tsunami had formed in the Indian Ocean, NOAA officials tried to get the message out to other nations not a part of its Pacific warning system to alert them of the possibility of a tsunami. (NOVA and Indian Ocean Tsunami)